Add keywords
If you gained no results or too few results, you can add one
or more related keywords with or operator. It makes your search
more hittable. e.g., tex or ptex or latex or latex2e
If you gaind too many results, you can add one or more
related keywords with and
operator. It makes your search more limited. e.g., latex and dvi2ps and eps

Try substring matching
If you gained no results or too few results, you can try
substring matching.
You can specify tex* to
search for terms which begin with
tex (e.g., tex,
texi2html,
texindex, text).
You can specify *tex to
search for terms which terminated with tex (e.g.,
bibtex,
jlatex, latex,
platex, ptex, vertex).
You can specify *tex* to
search for terms which contain tex (many).

You tried phrase searching but it hit documents which
didn't contain your phrase.
It's a defect of Namazu. Precision of phrase searching is
not 100 %, so it cause wrong results occasionally.

You want to use and,
or or not as ordinary keywords
You can surround them respectively with double quotes like "..." or braces like {...}.